So now computers in LAN could be SNAT to one of two IP 10.0.1.2,
10.0.2.2 to go out , but how to control what SNAT to chose .

The ideal of mine is writing a programe loadbalancing for
n line ADSL contact to one PC as gateway , and my start is control what SNAT to
use . If you have some idea or experience of if , help me please